Shielding for a particle accelerator

ABSTRACT

In the apparatus disclosed herein, the accelerating tube of a particle accelerator is shielded against the emission of X-rays by a shield which comprises particles of lead oxide supported in a semi-insulating rubber matrix.

United States Fatent 1151 3,675,061

Harrison July 4, 1972 [54] SHIELDING FOR A PARTICLE 2,404,225 7/1946 Green ..252/478 X ACCELERATOR 3,148,160 9/1964 Malm et a1... .252/478 3,328,618 6/1967 Wilson r r .313/63 [72] Inventor: Stanley llarrlson, Bedford, Mass. 3,409,776 1 H1968 Pipher at 11 1 50/108 [73] Assignee: KEV Electronics Corporation, Wilming- 3,473,064 10/1969 Herb "328/233 x on Mass 3,508,059 4/1970 .Vanderpool.... ..250/108 X [22] Filed: June 1 1969 Primary Examiner-Roy Lake [2]] Appl 274 Arsistant ExaminerPalmer C. Demeo Attorney-Kenway, Jenney & Hildreth [52] U.S. Cl ..313/63,250/108,252/478, [57] ABSTRACT 315/85, 328/233 [51] Int. Cl. ..l-l05h 5/00 In the apparatus disclosed herein, the accelerating tube f a Field of S r h... -....313/63; 315/85; 250/108; particle accelerator is shielded against the emission of X-rays 328/233; 252/478 by a shield which comprises particles of lead oxide supported in a semi-insulating rubber matrix.

[56] References Cited UNITED STATES PATENTS 3 Claims, 1 Drawing Figure 2,145,727 1/1939 Lloyd, Jr. ..313/63 UX ll l3 '9 5 A W iii H P'A'TEMEnJuL 41972 3 ,75 .061

INVENTOR BY KWWJIW ALLM ATTORNEYS SHIELDING FOR A PARTICLE ACCELERATOR BACKGROUND OF THE INVENTION This invention relates to particle accelerators and more particularly to such an accelerator which is shielded against the emission of X-rays.

Heretofore, particle accelerators have been used chiefly for scientific research and medical purposes, e.g., as neutron sources. In such applications, the accelerators have typically been installed in a vault which provides shielding against both the high-energy particles provided by the machine and also any X-rays which may be emitted by virtue of the operation of the machine. The generation of X-rays may occur when electrons, released by random collisions of the particles being accelerated with residual vapor molecules or with parts of the machine, are accelerated back up the accelerator tube with enough energy to generate X-rays upon collision with metal parts at the source end of the accelerator tube.

At the present time, however, particle accelerators are being proposed for use in applications of a commercial manufacturing nature, e.g., for the fabrication of semiconductor electronic components by ion implantation. In such applications, shielding is highly necessary for worker safety but vault type shielding is impractical.

As is understood, it is highly desirable that radiation shielding be placed as close to a radiation source as possible in order to reduce the amount of shielding material needed. The reduction in amount of shielding material needed occurs because the attenuation of radiation provided by a given shield depends essentially only upon its thickness and not upon the intensity of the ion flux impinging upon the shield. When dealing with the accelerating tube of a particle accelerator, however, shielding materials cannot be freely introduced since they may disrupt the high level potential gradients which are necessary to produce acceleration and focusing of the particles being accelerated.

Among the several objects of the present invention may be noted the provision of a shield adapted for use around the accelerating tube of a particle accelerator; the provision of such a shield which does not interfere with the normal operation of the accelerator; the provision of such a shield which does not initiate breakdowns along the tube; the provision of such a shield which substantially attenuates X-ray emissions from a particle accelerator; the provision of such a shield which is substantially continuous, i.e., without gaps; and the provision of such a shield which is relatively simple and inexpensive.

Other objects and features will be in part apparent and in part pointed out hereinafter.

SUMMARY OF THE INVENTION Briefly, the present invention is useful in a particle accelerator of the type in which an acceleration tube extends between a source and a utilization means which are maintained at widely differing voltage levels. The inventive improvement comprises a shield which surrounds the tube and which comprises particles of a material including a heavy metal supported in a semi-insulating matrix.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a side elevation, partially in section, of a particle accelerator having an acceleration tube shielded in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, the particle accelerator illustrated there may be described generally as comprising an ion source 11 and a target or utilization means, designated generally as 13, which are supported on a base 14. The source 1 1 and target 13 are linked or joined by an acceleration tube, designated generally as 15. The source and the target are maintained at widely differing voltage levels by a suitable source, not shown, so that a potential gradient, suitable for particle acceleration, exists across tube 15. Typically, the utilization means or target 13 is maintained at ground potential while the source 11 is maintained at a voltage level suitable for applying the desired acceleration to the particles, e. g., ions, under consideration. Ion source 11 may be of any conventional construction and thus is not illustrated in detail herein.

In order to provide a uniform field gradient for accelerating ions emitted by the source 11, acceleration tube 15 is constructed by providing a plurality of conductive field-grading rings 17, e.g., constructed of aluminum, which are separated by insulating, e.g., glass, spacers 19. The grading rings 17 are typically maintained at voltage levels which are graded from the potential of source 11 to that of the target 13 by means of a resistive voltage divider (not shown). Shielding around the ends of the accelerator tube may be provided by essentially conventional solid lead shield elements.

In accordance with the present invention, the emission of X- rays from the accelerating tube 15 itself is substantially reduced by means of a shield 21 which surrounds the accelerating tube from the source 11 to the utilization means 13 A preferred material for shield 21 is black rubber filled with lead oxide. This material is available commercially from a variety of sources for use in X-ray laboratory aprons. A sheet of this material may be wrapped around the accelerating tube 15 to form a shielding enclosure or tube 21.

The lead oxide, since it contains a substantial proportion of the heavy metal lead, i.e., an element of relatively high atomic number, is quiteefiective in attenuating X-rays emitted from the region of the accelerating tube 15. Further, the lead oxide is substantially insulating and thus does not, of itself, substantially disrupt the electrostatic fields which are essential to the proper operation of the accelerator. Further, while the rubber which constitutes the matrix of support for the lead oxide particles is generally an insulator, it is not such a good insulator that substantial surface charges can accumulate. As is understood, the accumulation of surface charges on an insulator adjacent the accelerating tube 15 can disrupt the electrostatic field and lead to harmful breakdowns along the surface of the tube or the shield. While black rubber as employed in shield 21 is not so conductive as to be classed as a semiconductor, neither is it such a perfect insulator as is glass. Thus, for the purpose of this application, materials which are essentially in sulators but which will not hold substantial local charge distributions, are referred to as semi-insulators.

Preferably, the conductivity of the shield is of a somewhat lower order of magnitude as compared with the conductivity of the resistive voltage divider which establishes the potentials on the various grading rings 17. Thus, the shield 21 may be in direct contact with the grading rings 17 as well as the source and the utilization means and the conductivity of the shield will not interfere with the voltage-dividing operation but will prevent the localized accumulation of static charges on the surface of the shield. In this way a continuous shield, i.e., without gaps, may be readily provided.

While lead oxide in black rubber has been specified by way of example, it will be understood that other types of heavy metal particles may be used, e.g., oxides of tungsten, and other semi-insulating matrix materials may be used.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a particle accelerator of the type in which an acceleration tube having alternate conductive and insulating segments extends between a source and a utilization means which are maintained at widely differing voltage levels, said conducting segments being maintained at respective graded voltage levels therebetween, the improvement comprising a shield which overlies said tube and is in electrical contact with said conductive segments and which comprises particles of an oxide of a heavy metal supported in a semi-insulating matrix, the potential difference between adjacent conductive segments being distributed relatively uniformly along the portion of said shield extending therebetween.

2. In a particle accelerator of the type in which an acceleration tube comprising a series of metallic field grading rings separated by insulating spacers extends between a source and a utilization means which are maintained at widely differing voltage levels and the rings are maintained at intermediate voltage levels, the improvement comprising an enclosure of lead oxide filled rubber around said accelerating tube in electrical contact with said rings, the potential differences therebetween being distributed relatively uniformly along said enclosure.

3. In a particle accelerator of the type in which an acceleration tube extends between a source and a utilization means which are maintained at differing voltage levels and which tube includes a series of field grading rings between said source and utilization means, the improvement comprising a shield which surrounds said tube from said source to said utilization means and is in electrical contact with said, source, said utilization means and said rings and which comprises lead oxide particles supported in a rubber matrix, the potential difference between said source and said utilization means being distributed relatively uniformly along said shield. 

1. In a particle accelerator of the type in which an acceleration tube having alternate conductive and insulating segments extends between a source and a utilization means which are maintained at widely differing voltage levels, said conducting segments being maintained at respective graded voltage levels therebetween, the improvement comprising a shield which overlies said tube and is in electrical contact with said conductive segments and which comprises particles of an oxide of a heavy metal supported in a semi-insulating matrix, the potential difference between adjacent conductive segments being distributed relatively uniformly along the portion of said shield extending therebetween.
 2. In a particle accelerator of the type in which an acceleration tube comprising a series of metallic field grading rings separated by insulating spacers extends between a source and a utilization means which are maintained at widely differing voltage levels and the rings are maintained at intermediate voltage levels, the improvement comprising an enclosure of lead oxide filled rubber around said accelerating tube in electrical contact with said rings, the potential differences therebetween being distributed relatively uniformly along said enclosure.
 3. In a particle accelerator of the type in which an acceleration tube extends between a source and a utilization means which are maintained at differing voltage levels and which tube includes a series of field grading rings between said source and utilization means, the improvement comprising a shield which surrounds said tube from said source to said utilization means and is in electrical contact with said source, said utilization means and said rings and which comprises lead oxide particles supported in a rubber matrix, the potential difference between said source and said utilization means being distributed relatively uniformly along said shield. 